GB2161609A

GB2161609A - Optical fibres

Info

Publication number: GB2161609A
Application number: GB08417663A
Authority: GB
Inventors: Richard Edward Epworth; Steven Wright; Roger Brambley; David Smith
Original assignee: Stc PLC; STC PLC
Current assignee: Stc PLC; STC PLC
Priority date: 1984-07-11
Filing date: 1984-07-11
Publication date: 1986-01-15
Also published as: GB2161609B; ES545072A0; GB8417663D0; US4781428A; ES8703643A1; AU4427485A; FR2567654A1; JPS6138912A

Description

1 GB2161609A 1

SPECIFICATION

Optical fibres This invention relates to optical fibres and in 70 particular to devices and methods for selec tively tapping optical energy from an optical fibre.

According to one aspect of the present invention there is provided a device for selec tively tapping optical energy from an optical fibre including means for causing spatially periodic deformation of the fibre and coupling of optical energy between modes of the opti cal fibre at a predetermined optical frequency or narrow band of optical frequencies, and means for removing from the fibre the optical energy at the predetermined optical frequency or narrow band of optical frequencies.

According to another aspect of the present invention there is provided a method of selectively tapping optical energy from an optical fibre including the steps of causing spatially periodic deformation of a section of the fibre and coupling of optical energy between modes of the optical fibre at a predetermined optical frequency or narrow band of optical frequencies, and removing from the fibre the optical energy at the predetermined optical frequency or narrow band of optical frequencies.

Embodiments of the invention will now be described with reference to the accompanying drawings, in which Figure 1 shows, schematically, an embodiment of a tunable device for tapping optical energy from an optical fibre, and Figure 2 shows a graph of power coupled versus wavelength for different grating peri- ods.

In our co-pending Application Nos.

8222371 (Serial No.) (R.E. Epworth- V.A. Handerek 24-1) and 8405853 (Serial No.) (R.E. Epworth-R.J. Brambley 26- 1) there are described the uses of sections 110 of optical fibres as sensing elements. A distur bance to be sensed, for example pressure fluctuations due to an acoustic wave, is em ployed to cause coupling between separate modes of a fibre section through an appropri- 115 ate transducer structure.

As described in the above-mentioned copending applications for sensor purposes the optical fibre is disposed relative to a support, for example, a grating or a cylindrical member whose diameter varies in a periodic manner, and the perturbation to be sensed causes the fibre to be pressed against the support and deformed, the amount of deformation achieved being related to the strength of the perturbation.

It is now proposed to employ controlled deformation of an optical fibre in order to selectively (by optical frequency) tap optical energy from an optical fibre. For example, a narrow frequency band of optical energy may be selectively tapped from a single mode fibre. As illustrated in Fig. 1 this is achieved by deforming a single mode fibre 1 by pressing it against a solid grating 2. The pressing means is indicated as a pressure plate 3 to which a load 1 is applied. The periodic deformation of the optical fibre 1 thus achieved will cause strong coupling between the guided mode (core mode) and the unguided cladding modes only when the spatial period or pitch of the deformation matches the beat length between the guided mode and the cladding modes. Coupling occurs between the guided mode and the cladding modes only at the optical frequency at which the inter-mode length matches the pitch of the fibre deformation. This frequency may be tuned by using different gratings of different pitches or by rotating the orientation of the grating with respect to the fibre, that is changing the angle 0. Fig. 2 illustrates the amount of power coupled to the cladding by a short grating versus wavelength for different grating peri- ods. The cladding light may be extracted by a suitable cladding mode tap 4 and detected by a photodetector 5. Examples of cladding mode taps are described in our British Patent Specifications No. 1590082 (R.E. Epworth

17) and in No. 1596869 (J.G. Farrington-T. Bricheno 8-1). Alternatively if a band stop filter is required then the tap 4 would be replaced by a region of cladding mode stripping material.

In the case of multimode optical fibre, controlled deformation of the fibre may be employed to achieve narrow band coupling between guided modes. Use of a polarisation maintaining fibre allows wavelength selective coupling between polarisation states and subsequent splitting of a required band may be achieved by means of a polarisation selective tap.

By varying the pressure between the fibre and the grating the extent of coupling (filter coupling) may be varied or switched off. Similarly, the filter shape may be tailored by varying the pressure across the grating.

The invention thus provides a tunable optical fibre wavelength demultiplexer. It is suitable for channel dropping in single mode systems. Multiple channel drops require several such devices. An arrangement comprising a sequence of such selective tapping devices can be envisaged, forming the multichannel wavelength multiplexer/demultiplexer, each device being associated with a respective optical frequency or narrow band of optical frequencies. The fibre itself is not permanently affected and does not need to be cut to install the device therein. The device may be cornprised as a clip-on device in order to act as a temporary monitor during optical fibre installation. Very narrow bandwidth signals may be extracted by subjecting the fibre to periodic 2 GB 2 161 609A 2 deformation over a very long length simply by winding it on a splined mandrel. Examples of such splined mandrel arrangements are described in the copending Application No. 8222371.

Claims

1. A device for selectively tapping optical energy from an optical fibre including means for causing spatially periodic deformation of the fibre and coupling of optical energy between modes of the optical fibre at a predetermined optical frequency or narrow band of optical frequencies, and means for removing from the fibre the optical energy at the predetermined optical frequency or narrow band of optical frequencies.

2. A device as claimed in claim 1 and for use with single mode optical fibre having a core and cladding thereon, wherein said deformation causing means serves to couple optical energy between the guided mode in the core and a mode or modes in the cladding, and said optical energy removing means serves to remove optical energy from the cladding.

3. A device as claimed in claim 2 and comprised as a band stop filter, the optical energy removing means comprising a region of cladding mode stripping material through which the fibre is passed.

4. A device as claimed in claim 2 and comprised as a wavelength demultiplexer, the optical energy removing means comprising a cladding mode tap.

5. A device as claimed in any one of the preceding claims wherein said means for causing spatially periodic deformation of the fibre comprises a mechanical grating and means for pressing the fibre there against.

means for causing spatially periodic deformation of the fibre comprises a splined mandrel around which the fibre is wound.

12. A method of selectively tapping opti- cal energy from an optical fibre including the steps of causing spatially periodic deformation of a section of the fibre and coupling of optical energy between modes of the optical fibre at a predetermined optical frequency or narrow band of optical frequencies, and removing from the fibre the optical energy at the predetermined optical frequency or narrow band of optical frequencies.

13. A method as claimed in claim 12 wherein said optical fibre is a single mode optical fibre having a core and cladding thereon, and said deformation causing step serves to couple optical energy between the guided mode in the core and the cladding, and wherein said optical energy removing step serves to remove optical energy from the cladding.

14. A method as claimed in claim 12 wherein said optical fibre is a multimode fibre and said deformation causing step serves to couple optical energy between guided modes thereof.

15. A device for selectively tapping optical energy from an optical fibre substantially as herein described with reference to the accompanying drawings.

16. A method of selectively tapping optical energy from an optical fibre substantially as herein described with reference to the accompanying drawings.

17. A multichannel wavelength multiplex- er/demultiplexer comprising a sequence of devices as claimed in any one of claims 1 to 11 or 15 each device being associated with a

6. A device as claimed in claim 5, wherein 105 respective optical frequency or narrow band of the pressure by which the pressure is urged optical frequencies.

against the grating is variable in a controlled manner.

7. A device as claimed in claim 5 or claim 6, wherein the predetermined optical fre quency or band of optical frequencies is deter mined by the relative orientation of the grat ing and the fibre, the device being tunable by adjusting said relative orientation.

8. A device as claimed in claim 1 and for use with multimode fibre, wherein said deformation causing means serves to couple optical energy between guided modes thereof.

9. A device as claimed in claim 8 and for use with polarisation maintaining fibre whereby coupling is achieved between polarisation states and wherein the optical energy removing means comprises a polarisation selective tap.

10. A device as claimed in any one of the preceding claims and comprised as a clip-on device for the use in monitoring an optical fibre during installation thereof.

11. A device as claimed in any one of claims 1 to 4 or claims 8 to 9 wherein said Printed in the United Kingdom for Her Majesty's Stationery Office. Dd 8818935, 1986. 4235. Published at The Patent Office. 25 Southampton Buildings, London, WC2A lAY. from which copies may be obtained.